The present invention concerns a low-pressure process for preparing an iron-based powder. More specifically, the invention concerns an annealing process for producing a low-oxygen, low-carbon iron or steel powder.
Annealing of iron powders is of central importance in the manufacture of powder metallurgical powders.
Previously known processes aiming at the production of low-oxygen, low-carbon iron-based powder are disclosed in e.g., U.S. Pat. Nos. 3,887,402; 4,448,746 and 4,209,320.
U.S. Pat. No. 3,887,402 concerns a process for the production of high density steel powders, wherein a molten stream of low carbon steel or low carbon alloy steel is atomised by high pressure water jet or inert gas jet to form powders, and after drying, the powders are heated in such inert gas as nitrogen or argon, whereby the reduction, decarburisation and softening of the powders are simultaneously carried out.
U.S. Pat. No. 4,448,746 concerns a process for the production of an alloyed steel powder having low amounts of oxygen and carbon. In this process, the amount of carbon of an atomised powder is controlled by keeping the powder in a decarburising atmosphere, which comprises at least H2 and H2O gases during certain periods of treatment, which are determined by temperature and pressure conditions. The amount of oxygen of the starting powder is essentially the same or somewhat lower than that of the annealed powder.
U.S. Pat. No. 4,209,320 discloses a process for the preparation of low oxygen iron-base metallic powder by using induction heating. In order to obtain powders having both a low oxygen and a low carbon content this patent teaches that so called rough reduced iron powders obtained by reducing mill scale with coke should be used. If the raw powder is a water-atomised powder high carbon levels are obtained.
Another process for producing steel powders having low amounts of oxygen and carbon is disclosed in the co-pending application PCT SE 97/0129.
The present invention concerns an alternative process for the preparation of steel powders having low amounts of oxygen and carbon or more specifically less than 0.25% by weight of oxygen and less than 0.01% by weight of carbon.
A distinguishing feature of the new process is it provides simple and effective process monitoring and that it can be carried out in a conventional batch furnace, which is preferably heated by direct electrical or gas heating even though it is possible to perform the process by induction heating.
Another distinguishing feature is that the process is carried out at low pressure.
In brief, the process according to the invention includes the following steps
a) water-atomising a raw powder essentially consisting of iron and optionally at least one alloying element selected from the group consisting of chromium, manganese, copper, nickel, vanadium, niobium, boron, silicon, molybdenum and tungsten and having a carbon content between 0.1 and 0.9, preferably between 0.2 and 0.7% by weight and an oxygen/carbon weight ratio of about 1 to 3, preferably between 1 and 1.5 and at most 0.5% of impurities;
b) charging a gas tight furnace with the powder in an essentially inert gas atmosphere and closing the furnace;
c) increasing the furnace temperature to a temperature between 800 and 1350xc2x0 C.,
d) monitoring the increase of the formation of CO gas and evacuating gas from the furnace when a significant increase of the CO formation is observed; and cooling the powder when the increase of the formation of CO gas diminishes.